Brontornis or 'Thunder bird' was one of the largest flightless birds that ever lived. It is one of the "Terror Birds" known for their large size and predatory lifestyle, and more precisely to the subfamily Brontornithinae, which contained extremely large and very heavyset forms.

Brontornis stood about 2.8 m (9.2 ft) tall and more robustly built. It is the heaviest species of phorusrhacid that weighed in the neighborhood of 350–400 kg (770–880 lb).This makes it the third-heaviest bird ever that lived, behind Aepyornis maximus and Dromornis stirtoni.